C# in Depth

50 CHAPTER 2 Core foundations: building on C# 1Now that we’ve got the basic idea of what reference types and value types are about,we’ll look at a few of the most important details.2.3.2 Value and reference type fundamentalsThe key concept to grasp when it comes to value types and reference types is what thevalue of a particular expression is. To keep things concrete, I’ll use variables as themost common examples of expressions—but the same thing applies to properties,method calls, indexers, and other expressions.As we discussed in section 2.2.1, most expressions have types associatedwith them. The value of a value type expression is the value, plain andsimple. For instance, the value of the expression “2+3” is just 5. The valueof a reference type expression, however, is a reference. It’s not the objectthat the reference refers to. So, the value of the expression String.Empty is not an empty string—it’s a reference to an empty string. In everydaydiscussions and even in documentation we tend to blur this distinction.For instance, we might describe String.Concat as returning “astring that is the concatenation of all the parameters.” Using very precise terminologyhere would be time-consuming and distracting, and there’s no problem so long aseveryone involved understands that it’s only a reference that is returned.To demonstrate this further, consider a Point type that stores two integers, x and y.It could have a constructor that takes the two values. Now, this type could be implementedas either a struct or a class. Figure 2.3 shows the result of executing the followinglines of code:Key point!Point p1 = new Point(10, 20);Point p2 = p1;The left side of figure 2.3 indicates thevalues involved when Point is a class (areference type), and the right side showsthe situation when Point is a struct (avalue type).In both cases, p1 and p2 have thesame value after the assignment. However,in the case where Point is a referencetype, that value is a reference: bothp1 and p2 refer to the same object. WhenPoint is a value type, the value of p1 is thewhole of the data for a point—the x andWhen Point isa value typey values. Assigning the value of p1 to p2 copies all of that data.The values of variables are stored wherever they are declared. Local variable valuesare always stored on the stack, 9 and instance variable values are always stored whereverp1refp2refx10When Point is areference type20x10x10Figure 2.3 Comparing value type andreference type behaviors, particularlywith regard to assignmentyp1p1y20y209This is only totally true for C# 1. We’ll see later that local variables can end up on the heap in certain situationsin C# 2 and 3.Licensed to Rhona Hadida